1. Field of the Invention
The present invention relates to metal nitride thin films. In particular, the invention concerns a method of growing tungsten nitride thin films by Atomic Layer Deposition (referred to as ALD hereinafter).
2. Description of Related Art
The integration level of components in integrated circuits is increasing, which rapidly brings about a need for a decrease of the size of components and interconnects. Design 15 rules are setting the feature sizes to ≦0.2 μm. Complete film coverage on deep bottoms and vias is bard to obtain.
Integrated circuits contain interconnects which are usually made of aluminium or copper. Especially copper is prone to diffusion to the surrounding materials. Diffusion affects the electrical properties of the circuits and active components may malfunction. The diffusion of metals from interconnects into active parts of the device is prevented with an electrically conductive diffusion barrier layer. Favored diffusion barriers are, e.g., amorphous transition metal nitrides, such as TiN, TaN and WN. The nitrides can be non-stoichiometric because nitrogen is located in interstitial position of the lattice.
In the Chemical Vapor Deposition method (referred to CVD hereinafter), the source materials arc typically fed to reaction space together, and they react with each other when brought into contact with the substrate. It is also possible to feed one source material containing all desired reactant species to a CVD reactor, and heat it almost to a point where it decomposes thermally. When the heated gas contacts the substrate surface, a cracking reaction occurs and a film is grown. As is apparent from the above discussion, in CVD the concentration of the different source materials in the reaction space determines the growth of the film.
Atomic Layer Deposition (ALD) and, originally, Atomic Layer Epitaxy (ALE) is an advanced variation of CVD. The method name was changed from ALE into ALD to avoid possible confusion when discussing about polycrystalline and amorphous thin films. The ALD method is based on sequential self-saturated surface reactions. The method is described in detail in U.S. Pat. Nos. 4,058,430 and 5,711,811. The reactor design benefits from the usage of inert carrier and purging gases which makes the system fast.
The separation of source chemicals from each other by inert gases prevents gas-phase reactions between gaseous reactants and enables self-saturated surface reactions leading to film growth which requires neither strict temperature control of the substrates nor precise dosage control of source chemicals. Surplus chemicals and reaction byproducts are always removed from the reaction chamber before the next reactive chemical pulse is introduced into the chamber. Undesired gaseous molecules are effectively expelled from the reaction chamber by keeping the gas flow speeds high with the help of an inert purging gas. The purging gas pushes the extra molecules towards the vacuum pump used for maintaining a suitable pressure in the reaction chamber. ALD provides an excellent and automatic self-control for the film growth.
ALD has recently been used for depositing single layers of titanium nitride TiN (H. Jeon, J. W. Lee, J. H. Koo, Y. S. Kim, Y. D. Kim, D. S. Kim, “A study on the Characteristics of TiN Thin Film Deposited by Atomic Layer Chemical Vapor Deposition method”, AVS 46th International Symposium, abstract TF-MoP17, http://www.vacuum.org/symposium/seattle/technical.html, to be presented Oct. 27, 1999 in Seattle, USA).
According to Hiltunen et al. NbN, TaN, Ta3N5, MoN and Mo2N can be grown by ALD using metal halogenides as source chemicals (L. Hiltunen, M. Leskelä, M. Mäkelä, L. Niinistö, E. Nykänen, P. Soininen, “Nitrides of Titanium, Niobium, Tantalum and Molybdenum Grown as Thin Films by the Atomic Layer Epitaxy Method”, Thin Solid Films, 166 (1988) 149-154). The use of additional zinc vapour during the deposition has decreased the resistivity of the nitride film either by increasing the metal/nitrogen ratio or by removing oxygen from the films.
J. W. Klaus has disclosed a process for growing tungsten nitride films using an ALD method (J. W. Klaus, “Atomic Layer Deposition of Tungsten and Tungsten Nitride Using Sequential Surface Reactions”, AVS 46th International Symposium, abstract TF-TuM6, http://www.vacuum.org/symposium/seattle/technical.html, to be presented Oct. 26, 1999 in Seattle, USA). In the process of the publication, tungsten nitride W2N is grown from WF6 and NH3.
In the art, tungsten compounds have been reduced by using hydrogen (H2) U.S. Pat. No. 5,342,652 and EP-A2-899 779), silanes, such as SiH4 (U.S. Pat. No. 5,691,235) and chlorosilanes, such as SiHCl3 (U.S. Pat. No. 5,723,384).
There are, however, drawbacks related to these prior art methods. Silanes may also react with WF6, thus forming tungsten silicides, such as WSi2. Hydrogen can reduce a tungsten compound into tungsten metal which has too low vapor pressure for being transported in gas phase onto substrates. Traditional CVD processes may leave significant amounts of impurities in thin films, especially at low deposition temperatures.